A115015 - OEIS

%I #7 May 09 2014 01:44:30

%S 1,3,6,10,15,21,21,29,38,48,59,71,84,84,99,115,132,150,169,189,189,

%T 211,234,258,283,309,336,336,365,395,426,458,491,525,525,561,598,636,

%U 675,715,756,756,799,843,888,934,981,1029,1029,1079,1130,1182,1235,1289,1344,1344

%N Sum_{i=1..n} (gcd(7,i)=1) i.

%F a(-1-n) = a(n). - _Michael Somos_, Jul 12 2006

%o (PARI) a(n)=binomial(n+1,2)-7*binomial(n\7+1,2) /* _Michael Somos_, Jul 12 2006 */

%o (PARI) a(n)=sum(i=1,n, i*(1==gcd(7,i))) /* _Michael Somos_, Jul 12 2006 */

%Y Cf. A115012.

%K nonn

%O 1,2

%A _N. J. A. Sloane_, Feb 24 2006